The present invention relates generally to the fabrication of Bragg gratings and, more specifically, to the fabrication of Bragg gratings in optical waveguides.
Increasing demands are placed on communication networks as the number of customers grows and as the need to offer more complex services expands. In optical networks, for example, wavelength division multiplexing (WDM) is used to transmit the massive amounts of information passing through the optical network. WDM technology enables information to be transmitted through optical fibers and optical devices in bit streams, each superimposed on light of a different wavelength. A WDM system, for example, may have a single fiber communicating 256 channels, or individual signals, existing at 256 slightly different wavelengths, each channel representing a different user or information stream. To perform routing, switching, multiplexing, demultiplexing, and add/drop functions on these individual signals, wavelength-selective devices are used to selectively affect signals at different wavelengths.
Fiber Bragg gratings are often used in WDM optical networks to form wavelength-selective devices. Fiber Bragg gratings function as optical filters in which nearly all of the light within a desired narrow bandwidth range is filtered while nearly all of the light outside this range is transmitted. They have been used in place of interference-based filters like thin film filters and arrayed waveguides. Despite their use, fiber Bragg gratings do restrict optical network design and implementation.
Optical networks are notoriously difficult to achieve and slow to reconfigure. Reconfigurations, however, are desirable to add customers and services, as well as to manage traffic changes within the network. For example, it is desirable to have tunable components within an optical network that transmit or filter certain frequencies in a controllable manner. In other words, it is desirable to have a single tunable wavelength-selective device to replace multiple wavelength-selective devices. Single-device, fiber Bragg grating filters have been shown in which tuning is achieved by applying stress to the fiber or in which tuning is achieved by heating the fiber. These solutions are difficult to implement and limit optical device design and dimensions.
As an alternative to fiber-based filters and other optical devices, planar lightwave circuits (PLCs) are a class of similarly-formed optical devices used in WDM systems. PLC technology is advantageous because it can be used to form small-scale components, such as arrayed-waveguide grating (AWG) filters, optical add/drop (de)multiplexers (ADMs), optical switches, as well as hybrid opto-electronic integrated devices. Such devices formed with optical fibers would typically be much larger. Further, PLC structures can be batch fabricated on a silicon wafer, as well. Further still, with PLC technology, large-scale integration is theoretically achievable, i.e., the combination of numerous devices onto a single substrate or die similar to the large-scale integration common in microprocessor design. Unfortunately, using PLC technology to form a large-scale integrated optical chip has proved difficult, and one of the main problems arises with forming Bragg gratings on a PLC.
Two techniques have been shown for forming Bragg gratings on PLC waveguides: a semiconductor etching technique and an ultra-violet (UV) exposure technique. The former process is susceptible to grating periodicity errors in Bragg gratings of larger grating lines. Also, the former technique makes forming apodized gratings very difficult, though apodized gratings are highly desired for WDM systems because they offer better side lobe suppression of wavelengths outside of the grating resonant bandwidth range.
The more common method of forming Bragg gratings in PLC devices is through an UV exposure process in which a spatially periodic interference pattern is exposed on the fiber or waveguide. Though, increasingly more prevalent, this method is still unsatisfactory.
The UV spot sizes used during the UV exposure are quite large, at least 50 xcexcm in diameter, and, as a result, a relatively large area of the PLC device is exposed during the UV exposure process. Any waveguides in the PLC device within the UV spot size will also be patterned during the exposure, an undesirable effect in many integrated optical devices. Thus, one avoids forming other waveguides near the waveguide in which the grating is to be formed. This limits the extent to which optical devices may be densely packed into a large-scale integrated optical chip. The problem also limits the fabrication of individual optical components. For example, with components formed of two waveguides within evanescent coupling contact, Bragg gratings cannot be written into one waveguide without writing a Bragg grating into the other.
Not only does the realization of Bragg gratings in PLC devices limit device size and complexity, thus far easily fabricated tunable Bragg gratings, though achievable in optical fibers, have not been shown in PLC devices. A common way to make silica waveguide circuits (often used in PLC fabrication) tunable is to place a metal heater directly above the waveguide. Changing the waveguide temperature via the heater causes a change in the refractive index of the waveguide, and this changes the optical path length of the waveguide, which in turn can be used to tune a waveguide device. A problem arises when trying to combine Bragg gratings with current thermal heaters.
In batch fabrication, the metal heaters are formed before the wafer of the PLC structure is cut, or diced, to allow easy metal deposition and etching of the metal layer. UV exposure is performed after the wafer is diced, because UV writing stages are too small to properly expose an entire wafer and because, in diced form, the individual dies may be placed in a hydrogen chamber to increase photosensitivity before the UV exposure step. Forming the metal heater directly above a waveguide before UV exposure, however, would prevent the formation of a Bragg grating on the waveguide, as the UV exposure radiation would be absorbed by the metal heater. As a result, easily fabricated tunable Bragg gratings have not been shown in PLC devices.